Why 20 amino acids?

It is generally accepted that primitive genetic codes employed fewer than 20 amino acids. In other words, biochemistry in a primitive cell was realized by using simplified proteins including only the amino acids that were considered to be used in primitive genetic codes. We wonder whether it is possible to construct such biochemical systems using simplified proteins. In this paper, we will describe previous studies about the number of amino acids in genetic codes and introduce our research in order to answer the question. In our recent study for construction of simplified genetic code, codons for amino acids are reassigned to alanine or serine to reduce a number of amino acids in the engineered code. Simplified genetic codes will provide an effective tool for construction of simplified proteins.